Some radio systems, including the 5 GHz Radio Local Area Networks (RLANs), sometimes referred to as Wireless Local Area Networks (WLANs), use radio spectrum bands that are shared with other services including RADARs. This sharing requires that the radio communications equipment must detect the operation of nearby RADAR systems, and stop using the channels if a RADAR signal is detected. The band sharing procedures are defined in the ITU-R WRC-03 Resolution 229 and ITU-R Recommendation M1652. These texts are supplemented by additional national and regional regulations for sharing and equipment testing (e.g. ETSI EN301-893 for the EU).
The method that has been endorsed by the ITU agreements to facilitate sharing between the radio communications and RADAR services is referred to as Dynamic Frequency Sharing (DFS) (e.g. ITU-R Recommendation M1652 and ETSI standard EN301-893). The ITU outlines a method and conditions for detecting periodic RADAR signals based on RADAR signal strength threshold, pulse width and periodicity.
The initial WRC-03 set an upper limit of the RADAR pulse width of about 20 microseconds which is less than the typical length of the radio communications signal bursts. This, in principle, enabled the radio communications to distinguish RADAR signals from radio communications signals and other noise. However, new RADAR technology is using pulses much longer than 20 microseconds, since these provide some advantages for the RADAR system for resolution, sensitivity and range. These longer pulses are about the same length as the typical radio communications signals making more difficult the distinction between communications signal collisions and the RADAR signals.
A radio communications receiver will see many signals in its band/channel that may mimic the RADAR pulse width and strength. These false signals occur due to RF noise or the transmissions from other radio communications devices in the band, and radio communications receivers have trouble distinguishing a real RADAR pulse sequence from this other noise and activity. These extra signals may be radio communications signals, noise (i.e. thermal and man-made artificial) or collisions between two or more radio communications signals or collisions between radio communications signals and RADAR signals. The decision that a RADAR signal has been detected is thus ambiguous. As a result, a significant number of pulses must be observed to reduce the false detection probability to an acceptable level for the radio communications operation.
The ITU method was designed to detect “conventional” RADAR systems by distinguishing the RADAR signals due to their pulse width and their periodicity. These characteristics are not shared by noise or other radio communications traffic. The ITU method is suitable for commercial RADAR systems that utilize traditional revolving beam antennas and have regular pulse emissions. However, many RADARs use a variety of pulse formats, durations and repetition intervals either for operational reasons or a desire to be covert (i.e. hard to detect). These systems are aperiodic and may also change their pulse formats, modulation and timing often and in a seemingly random way (see for example the description of the SENRAD: An Advanced Wideband Air-Surveillance Radar, Skolnik et al.; Naval Research Laboratory, IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 37, NO. 4 OCTOBER 2001, page 1163). Hence the radio communications receiver cannot use the periodicity and limited range of pulse widths as a reliable means to distinguish these RADAR signals
Therefore, it is desirable to provide a method and system that is capable of better detecting conventional, variable and covert RADAR signals and of providing more reliable radio communications service.